Thanks for the clarification. I'm not certain where on when I came across it but, I though that the RPL that is implemented in HP-28 and 48 series stood for Reverse Polish LISP. So, I thought LISP was a postfix-based notation. Or is it that the 28/48 series use LISP but a with Reverse Polish command structure?

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Or is it that the 28/48 series use LISP but a with Reverse Polish command structure?

RPL is Lisp-like in some respects, but it definitely is not Lisp, and it obviously uses Reverse Polish Notation.

Note that "RPL" doesn't officially stand for anything. Bill Wickes has stated that the developers never called it anything other than "RPL". There are several unofficial expansions, including "ROM-based Procedural Language".